1. Field of the Invention
The present invention relates to a servo control system. More particularly, it relates to a servo control system first carrying out coarse (rough) control and thereafter changing to and carrying out fine control.
The present invention is applicable to a variety of fine position control systems, for example, a magnetic disc system, an optical disc system, a printer, or an industrial robot system.
2. Description of the Related Art
Servo control systems enable fine position control and thus have been extensively employed for a variety of control systems. In a servo control system, coarse control is applied, for example, speed control for moving a controlled object in a controlled system to a certain position adjacent to a reference position. Thereafter, fine control is applied, for example, a position control for fine position control of the controlled object at the reference position.
For example, in a magnetic disc system, coarse control, specifically, speed control, is used for a seek operation: for moving a magnetic head from a cylinder (or track) to a target cylinder. Thereafter, fine control, specifically, a position control, is used for fine position control of the magnetic head to a permissible position error region of the target cylinder. Normally, the position control may be carried out by a PID control algorithm. Control parameters for the PID control are adjusted to optimize the positional control to a steady state. Accordingly, when changing from speed control to position control, there is unavoidably a considerable large overshoot and/or undershoot. Thus, a considerably long settling time is required. This lengthens the operation time of the magnetic head. This will be described in more detail later with reference to a specific example.